Prop-Packed Fractures- A Reality On Which Productivity Increase Can Be Predicted

Abstract

The placement of a propping agent in hydraulically created fractures is a more adequate basis for predicting the folds of increase after a job. This is based on the premise that all unpropped areas of the created fracture eventually heal. Thus, the penetration of the prop pack into the reservoir and the amount of fill-up in the fracture determine the stimulation results. INTRODUCTION PRESENT METHODS of predicting the results of stimulation treatments are based on the productivity index ratios developed by McGuire and Sikora(1) (Figure 1). The predicted folds of increase are in relation to the conductivity ratio before and after fracturing and the penetration of the producing formation. This assumes that the propping agent supports the entire area of the hydraulically created facture(2). Although the industry has recognized for some time that the propping agent neither penetrates the full length of the fracture nor gives complete fill-up, the complexities of describing the placement of the prop have prevented using this feature in the treatment design. Limited use has been made of the settling-rate method for describing the extent which the prop pack penetrates the vertical fracture. The quotient of the vertical extent of the fracture and the settling rate gives the maximum time the prop is suspended by the fluid. The product of this time and the velocity of the fluid in the fracture gives the maximum distance the prop penetrates the fracture. Stokes Law can be used to predict the settling rate in Newtonian fluids, but is not applicable for non-Newtonian or power-law fluids. Kerns(3) et al. and Babcock(4) et al. have made studies on the transport of prop in vertical fractures, introducing the term ?equilibrium velocity' to the industry. Equilibrium velocity is the minimum linear velocity required to keep the prop moving through the fracture. A comparison of the settling-rate method and the equilibrium-velocity method is illustrated in Figure Z. However, as is known, little has been done toward using these methods for predicting the placement of prop in the fracture. One reason for this is that most of the fluids employed in these studies have been Newtonian, and the majority of the fracturing fluids are non-Newtonian. For this reason, a study was undertaken to expand the available information on commonly used fracturing fluids, such as gelled water. Laboratory tests were run in a plexiglas model of fixed, but variable, fracture width. The simulated vertical linear fracture was 2 ft in height by 6 ft long. Fracture widths were variable from 0.1 to 0.5 inch. Prop-laden fluids passing through this fracture exhibited the same general phenomena. While pumping down the pipe, the prop is fairly evenly dispersed in the fracturing fluid. However, as the fluid 'turns the corner, going into the fracture, centrifugal force and gravity concentrate the prop in the fluid moving through the lower part of the fracture. The bulk of the prop moves along the bottom of the fracture, as a fluidized bed.

Experimental Research on the Proppant Transport Behavior in Nonviscous and Viscous Fluids

“…It is known from the work of predecessors that the sand dune's equilibrium height is related to the fluid viscosity, velocity, proppant concentration, and proppant size. 1,3,4 The advancing angle of the dune is similar to the repose angle of particles, which is related to the physical properties of particles and fluids (such as particle sphericity and surface roughness, and fluid viscosity), but not to fluid flow rate, sand concentration, and particle size. 38,39 The advancing angle of the dune is 32°and equals the repose angle of the proppant (in the air).…”

“…The equilibrium height of the dune in this case was about 10.15 cm, accounting for 67.7% of the height of slot. It is known from the work of predecessors that the sand dune’s equilibrium height is related to the fluid viscosity, velocity, proppant concentration, and proppant size. ,, The advancing angle of the dune is similar to the repose angle of particles, which is related to the physical properties of particles and fluids (such as particle sphericity and surface roughness, and fluid viscosity), but not to fluid flow rate, sand concentration, and particle size. , The advancing angle of the dune is 32° and equals the repose angle of the proppant (in the air). Furthermore, tail fluctuation will appear when the particles leave the top of the dune if the dune is high enough, and some particles can spin back.…”

“…The industry has recognized for some time that the proppant neither penetrates the full length of a fracture nor completely fills it , but forms a sand dune (or sand bed) in the fracture. The shape of the dune is governed by the rheology of fracturing fluids, properties of proppant, geometry of fractures, and pumping rate. − …”

“…Currently, the single-wing transparent parallel plates experimental devices used at the University of Texas, University of Alberta, University of California, China University of petroleum, Southwest Petroleum University, BJ Service Company, Sinopec Research Institute, and others − do not demonstrate a big difference compared with Kern’s and Babcock’s apparatuses, except for some improvements on the fracture wall and entrance. For the design of fracture entrance, Tong et al injected fluid from the top of the slot by gravity, Alderman and Wendorff and Brannon et al used a trumpet-shaped inlet to distribute fluid and pressure more evenly in the fracture; these two types of entrances did not take the influence of perforations into account. Clark, Fjaestad and Tomac, and Tran et al set the inlet as a small hole at the middle of the height and showed the dynamic flow process of carrying fluid entering the slot through the small hole.…”

Bed load proppant transport during slickwater hydraulic fracturing: Insights from comparisons between published laboratory data and correlations for sediment and pipeline slurry transport

“…A more detailed mathematical description of this process was provided by Schols and Visser (1974). Babcock et al (1967) and Alderman and Wendorff (1970) performed laboratory experiments in which they observed equilibrium bed height behavior. They developed correlations to predict the critical velocity at equilibrium.…”